Fixes the in-memory focused node stack so it can be used to find the currently-focused node. This is necessary for `closeFocusedNode` to work. Also fixes `validateFocusedNode` so it will set the first node to be focused if no focused node is available. Also cleans up leafOrder update pattern.
This PR is a large refactoring of the layout code to move as much of the
layout state logic as possible into a unified model class, with atoms
and derived atoms to notify the display logic of changes. It also fixes
some latent bugs in the node resize code, significantly speeds up
response times for resizing and dragging, and sets us up to fully
replace the React-DnD library in the future.
This PR swaps usage of the `uuid` library for the built-in
`crypto.randomUUID` function, which is available in both NodeJS and the
browser. The built-in function is around 12x faster than the `uuid`
library. The strings produced by the built-in function are fully
compatible with the UUIDv4 standard, so it's an easy switch.
